Copending U.S. patent application Ser. No. 653,931 discloses a method for accurately measuring mass airflow into an internal combustion engine without a mass airflow meter. The method employs a technique of iterative prediction and estimation to determine mass airflow into the engine in response to measures of various engine parameters and a set of model parameters. One implementation of the method is described below.
Various engine parameters are measured with various input devices at each time event, with k designating the present time event. The measured engine parameters include throttle position, TPS(k), engine speed, RPM(k), idle air control valve position, IAC(k), exhaust gas recirculation (EGR) valve position, EGR(k), air temperature, T(k), atmospheric pressure, ATM(k), and intake manifold absolute pressure, MAP(k). A mathematical model of the engine is predetermined and comprises the parameters: (i) a.sub.t, the MAP prediction model state coefficients, (ii) b.sub.t, the airflow prediction model state coefficients, (iii) c.sub.t, the MAP prediction model input coefficients, (iv) d.sub.t, the airflow prediction model input coefficients, and (v) h.sub.1 and h.sub.2, the MAP and mass airflow prediction model constants, respectively.
Manifold pressure predictions and mass airflow predictions, MAP.sup.P (k-t) and MAP.sup.P (k-t), comprising the vector X.sup.P (k), where: ##EQU1## and where i and j are system constants, are computed from previous estimations of manifold air pressure and mass airflow, MAP.sup.e (k-t) and MAF.sup.e (k-t), comprising vector X.sup.e (k), the measured engine parameters comprising vector U(k), and the model parameters comprising matrices A, B, and C. the vectors X.sup.e (k), U(k), and matrices A, B, and C are defined as follows: ##STR1## where e, m, n, and 1 are system constants. The prediction of manifold pressure and mass airflow is governed by the equation: EQU X.sup.P (k+1)=AX.sup.e (k)+BU(k)+C.
After the prediction step, an estimation of manifold pressure and mass airflow, MAP.sup.e (k-t) and MAF.sup.e (k-t), comprising vector X.sup.e (k) is computed according to the equation: EQU X.sup.e (k+1)=X.sup.P (k+1)+G(MAP(k+1)-MAP.sup.P (k+1)),
where G is a vector comprising manifold pressure and mass airflow estimator correction coefficients, G.sub.1,t and G.sub.2,t, respectively, such that: ##EQU2## where the estimator correction coefficients comprising G are determined through a method such as statistical optimization. The estimated mass airflow, MAF.sup.e (k+1), is used as an accurate measure of mass airflow into the engine which is necessary for supplying the appropriate air fuel ratio for the engine and other engine controls.
The accuracy of the software air meter can be limited if a bias in one of the input devices used to measure the engine parameters occurs or if there is an error in one of the model parameters. What is desired is a method of compensating for these biases and like biases in similar systems.